Imaging apparatus, control method of imaging apparatus, and program

ABSTRACT

An imaging apparatus includes an imaging element, a readout unit, and a signal processing unit. The imaging element includes pixels that have photoelectric conversion units for one micro lens. The readout unit performs a first readout operation on pixels included in a first region of the imaging element to read signals according to accumulated electric charges, and performs a second readout operation on pixels included in a second region different from the first region to read signals according to accumulated electric charges. The signal processing unit makes a correction for reducing noise levels to the signals readout by the readout unit. The signal processing unit makes the correction to the signals acquired from the first region such that differences between noise levels included in the signals read by the first readout operation and the second readout operation become smaller after the correction by the signal processing unit.

BACKGROUND OF THE INVENTION Field of the Invention

An embodiment relates to a technique for reducing noise in an imagesignal generated by an imaging apparatus.

Description of the Related Art

Japanese Patent Laid-Open No. 2013-068759 proposes an imaging element inwhich pixels having two photoelectric conversion units for one microlens are laid out on the entire surface. To detect a phase difference,it is necessary to perform a readout operation on the pixels with theplurality of photoelectric conversion units in such a manner as toobtain at least two signals with a parallax. To this end, there is apossible method by which to output a signal obtained by one of thephotoelectric conversion units and then output a signal obtained by theother photoelectric conversion unit, for example. Alternatively, thereis another possible method by which to first output a signal obtained byone photoelectric conversion unit and then add up signals obtained bytwo photoelectric conversion units to output the signal. However, thereadout operation for obtaining two signals requires about two times aslong as the time for an addition readout operation by which to read andadd up signals from the plurality of photoelectric conversion units forthe individual pixels. This takes a lot of time to realize phasedifference detection. Accordingly, Japanese Patent Laid-Open No.2013-068759 discloses that signal readout is performed in such a mannerthat switching takes place within one frame between the operation ofreading two signals from the pixels at predetermined periods and theoperation of reading and adding up the signals from the individualpixels. Then, performing focus control with the use of the read twosignals makes it possible to performance an image plane phase differenceautofocus (AF). Accordingly, focus control can be performed whilesuppressing increase in time for signal readout due to the image planephase difference AF.

However, to read signals of one frame by the use of the imaging elementdisclosed in Japanese Patent Laid-Open No. 2013-068759, when separatereadout and addition readout are selectively performed by line, there isan issue that the level of noise varies from line to line.

The reason will be briefly explained. In the case where one pixelincludes two photo diodes (PDs) to perform a pupil-dividing function,first, signals obtained by the individual PDs are output and then thesignals obtained by the two PDs are added up and output in lines inwhich signals for focus detection are read, whereas signals obtained bythe two PDs are merely added up and output in rows in which signals forimage generation are read. Since the signal output is performed twice inthe lines in which signals for focus detection are read, it takes longertime between making a reset and completing readout as compared to therows in which signals for image capture are read and the signals areoutput only once. This decreases the operating frequency range betweenthe signals and increases flicker noise. Accordingly, when focusdetection is conducted only in part of an image and signals for focusdetection are read only from the rows in that part, there arises anissue that the noise level varies between the lines in which focusdetection is conducted and the rows in which focus detection is notconducted.

In addition, in the case of reading separately the signals obtained bythe two PDs, for example, the signal for image capture is obtained byreading the single pixels twice and adding up the signals, readout noiseis superimposed twice. This noise issue caused by driving the imagingelement as described in Japanese Patent Laid-Open No. 2013-068759 hasnot been recognized at all.

SUMMARY OF THE INVENTION

For an imaging apparatus including an imaging element having pixels witha plurality of photoelectric conversion units, an embodiment workstowards suppressing influence by different noise components resultingfrom different reading methods.

According to an aspect of the present invention, an imaging apparatusincludes an imaging element including a plurality of pixels configuredto have a plurality of photoelectric conversion units for one microlens, a readout unit configured to perform a first readout operation onpixels included in a first region of the imaging element to read signalsaccording to accumulated electric charges, and perform a second readoutoperation, different from the first readout operation, on pixelsincluded in a second region different from the first region to readsignals according to accumulated electric charges, and a signalprocessing unit configured to make a correction for reducing noiselevels to the signals readout by the readout unit, wherein the signalprocessing unit makes the correction to the signals acquired from thefirst region such that differences between noise levels included in thesignals read by the first readout operation and the second readoutoperation become smaller after the correction by the signal processingunit.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments (with reference to theattached drawings).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an example of a schematicconfiguration of an imaging apparatus.

FIG. 2A is a planar view of a pixel having two PDs and FIG. 2B is aplanar view of a pixel having four PDs.

FIG. 3 is a diagram illustrating a schematic configuration of an imagingelement according to an embodiment.

FIG. 4 is a diagram illustrating an internal circuit configuration of apixel according to the embodiment.

FIG. 5 is a diagram illustrating timings for an addition readoutoperation by the imaging element according to the embodiment.

FIG. 6 is a diagram illustrating timings for a division readoutoperation by the imaging element according to the embodiment.

FIG. 7 is a diagram illustrating an example of a selective drive controlat the time of readout of one frame according to the embodiment.

FIG. 8 is a flowchart of a process by a first NR processing unitaccording to the embodiment.

FIGS. 9A to 9G are diagrams illustrating a filtering process by thefirst NR processing unit according to the embodiment.

FIG. 10 is a diagram illustrating an example of a configuration of asecond NR processing unit according to the embodiment.

DESCRIPTION OF THE EMBODIMENTS

Exemplary embodiments will be explained below in detail with referenceto the accompanying drawings. However, configurations of the followingembodiments are mere examples and not limited to the configurationsillustrated in the drawings.

First Embodiment <Configuration of an Imaging Apparatus>

FIG. 1 is a block diagram illustrating a schematic configuration of animaging apparatus of the embodiment including an image processing device11 that processes signals output from an imaging element 100.

In the configuration of FIG. 1, light flux entered via a lens unit 10forms an image on a light receiving surface of the imaging element 100.An object image formed on the light receiving surface is subjected tophotoelectric conversion into electric charges according to the amountof incident light at two PDs 21 a and 21 b of a pixel 20 of the imagingelement 100 illustrated in FIG. 2A, and the electric charges areaccumulated there. The electric charges accumulated in the PDs 21 a and21 b are sequentially read as voltage signals according to the electriccharges from the imaging element 100 based on a driving pulse providedby a timing generator 111 under an instruction from a control unit 109composed of a CPU. The control unit 109 instructs the timing generator111 to perform addition readout or division readout from individualrows. The addition readout and the division readout will be describedlater in detail.

The image signals read by the imaging element 100 are input into acorrelated double sampling (CDS)/gain control amplifier (AGC) circuit102. The CDS/AGC circuit 102 performs correlated double sampling toremove reset noise, gain adjustment, signal digitization. The CDS/AGCcircuit 102 outputs the image signals output from the imaging element100 to a sensor correction unit 103 based on the driving pulse providedby the timing generator 111. The CDS/AGC circuit 102 outputs imagesignals for focus detection output from the imaging element to a focussignal processing unit 104, and the details will be described later.

The sensor correction unit 103 makes a correction to remove theinfluence of image quality degradation in relation to the configurationof the imaging element 100. The target of the correction includes theinfluence of sensitivity failure of the PDs 21 a and 21 b (defectivepixel), for example. The target of the correction also includes theinfluence of shot noise and dark current on the PDs 21 a and 21 b and afloating diffusion (FD) region 423 illustrated in FIG. 4, and unevencharacteristics of a column amplifier in a readout circuit 303.

A first NR processing unit 105, as a portion of an application-specificintegrated circuit (ASIC) in which circuits with a plurality of specificfunctions is integrated, performs a noise reduction process (NR process)on image signals output from the sensor correction unit 103 to adjustthe amounts of noise in the individual lines resulting from thedifference between the addition readout operation and the divisionreadout operation. The process by the first NR processing unit 105 willbe described later in detail.

A signal processing unit 106, as another portion of the ASIC, subjectsthe image signals output from the first NR processing unit 105 tovarious kinds of image processing such as white balance correction,color level adjustment, false color suppression, high-frequencycomponent processing, gamma correction, shading correction, and the liketo generate image signals. The signal processing unit 106 also has asecond NR processing unit 1061 to perform a process for reducing noiseincluded in the image signals. The process by the second NR processingunit 1061 will be described later in detail. The first NR processingunit 105 and the signal processing unit 106 may not be provided ashardware, as the control unit 109 can perform according to thedescription in a program, to realize the same effects as the first NRprocessing unit 105 and the signal processing unit 106.

A display unit 107 is a display device such as an LCD, an organic EL, orthe like to display an image based on the image signals output from thesignal processing unit 106. In a recording mode to record the imagesignals, the processed image signals are sent from the signal processingunit 106 to a recording unit 108 and are recorded in a recording mediumsuch as an optical disc, a semiconductor memory, a magnetic tape, or thelike.

The focus signal processing unit 104 performs a publicly knowncorrelation calculation with a pair of focus detection signals outputfrom the CDS/AGC circuit 102 to determine defocus amount, and outputsthe determined defocus amount to the control unit 109. The control unit109 performs a focus control to drive a focus lens included in the lensunit 10 to a focus position based on the obtained defocus amount.

The control unit 109 performs a control to determine whether to performaddition readout or division readout from the individual rows asdescribed above, and exchanges information with the components of theimage processing device 11 to control the components. Further, thecontrol unit 109 powers on or off the apparatus, changes settings, makesrecords, and switches between auto focus (AF) control and manual focus(MF) control according to the input from an operation unit 110 operatedby the user. The control unit 109 also performs various functionsaccording to the user operation such as verification of recorded imagesand selection of a focus detection region.

<Schematic Configuration of the Imaging Element>

FIG. 2A is a planar view of a pixel 20 constituting a pixel section 301.As illustrated in FIG. 2A, the pixel 20 corresponds to one micro lens 21and has two photoelectric conversion units composed of PDs 21 a and 21b.

FIG. 2B illustrates a structure of four photoelectric conversion unitscorresponding to one micro lens 22 and composed of PDs 22 a, 22 b, 22 c,and 22 d. The following description is based on the structure in whichthe two PDs correspond to the one micro lens illustrated in FIG. 2A.However, the following description is also applicable to the structurein which three or more PDs correspond to one micro lens as illustratedin FIG. 2B.

FIG. 3 is a diagram illustrating a schematic configuration of theimaging element 100 according to the embodiment. As illustrated in FIG.3, the imaging element 100 has a pixel section 301 in which a pluralityof pixels is arranged two-dimensionally, a vertical scan circuit 302, areadout circuit 303, a horizontal scan circuit 304, and an outputcircuit 305. The pixel section 301 includes an effective region 301 a inwhich incident light is received and subjected to photoelectricconversion, an optically light-shielded vertical optical black (OB)region 301 b, and an optically light-shielded horizontal optical black(OB) region 301 c.

The vertical scan circuit 302 selects and controls an arbitrary pixelline from the pixel section 301. The readout circuit 303 reads thesignals output from the pixels in the line selected by the vertical scancircuit 302, and transfers the read signals to the output circuit 305under control of the horizontal scan circuit 304. The output circuit 305sends the signals to the outside of the imaging element 100.

FIG. 4 is an equivalent circuit diagram illustrating a configuration of,out of the plurality of pixels provided in the pixel section 301, thepixels in two adjacent rows (j-th row and (j+1)-th row) and two adjacentcolumns (i-th column and (i+1)-th column) and the readout circuit 303for the two columns (i-th column and (i+1)-th column).

A control signal ΦTXA(j) is input into the gate of a transfer switch 422a of pixels 420 in the j-th row, and a control signal ΦTXB(j) is inputinto the gate of a transfer switch 422 b of the same. A reset switch 424is controlled by a reset signal ΦR(j). The control signals ΦTXA(j) andTXB(j), the reset signal ΦR(j), and a row selection signal ΦS(j) arecontrolled by the vertical scan circuit 302. Similarly, pixels in the(j+1)-th row is controlled by control signals ΦTXA(j+1) and ΦTXB(j+1), areset signal ΦR(j+1), and a row selection signal ΦS(j+1).

A vertical signal line 427 is provided for each pixel column and isconnected to a power source 428 and transfer switches 430 a and 430 b ofthe readout circuit 303 provided in each column.

A control signal ΦTN is input into the gate of the transfer switch 430a, and a control signal ΦTS is input into the gate of the transferswitch 430 b. In addition, a control signal ΦPH output from thehorizontal scan circuit 304 is input into the gates of transfer switches432 a and 432 b. An accumulation capacitor unit 431 a accumulates theoutput from the vertical signal line 427 when the transfer switch 430 ais on and the transfer switch 432 a is off. Similarly, an accumulationcapacitor unit 431 b accumulates the output from the vertical signalline 427 when the transfer switch 430 b is on and the transfer switch432 b is off.

By turning on the transfer switches 432 a and 432 b in the i-th columnby a column selection signal ΦPH(i) from the horizontal scan circuit304, the outputs of the accumulation capacitor units 431 a and 431 b aretransferred to the output circuit 305 via different horizontal outputlines.

As readout operations for reading signals from the imaging element 100configured as described above, an addition readout operation (firstreadout operation) and a division readout operation (second readoutoperation) can be selectively performed. The addition readout operationand the division readout operation will be explained below withreference to FIGS. 5 and 6. In the embodiment, the switches are turnedon when the control signals are in H (high) state and are turned offwhen the control signals are in L (low) state.

<Addition Readout Operation> (First Readout Operation)

FIG. 5 illustrates timings for reading signals from the pixel in thej-th row of the imaging element 100 by the addition readout operation.At time T1, the reset signal ΦR(j) becomes H. Then, when the controlsignals ΦTXA(j) and ΦTXB(j) become H at time T2, the PDs 421 a and 421 bof the pixel 420 in the j-th row are reset. PDs 421 a and 421 billustrated in FIG. 4 correspond to the PDs 21 a and 21 b illustrated inFIG. 2, respectively.

When the control signals ΦTXA(j) and ΦTXB(j) become L at time T3, thePDs 421 a and 421 b start charge accumulation. Subsequently, when therow selection signal ΦS(j) becomes H at time T4, a row selection switch426 is turned on and connected to the vertical signal line 427, and asource follower amplifier 425 enters operational state.

When the reset signal ΦR(j) becomes L at time T5 and then the controlsignal ΦTN becomes H at time T6, the transfer switch 430 a is turned onto transfer a signal after reset cancel on the vertical signal line 427(noise signal) to the accumulation capacitor unit 431 a.

Then, the control signal ΦTN becomes L at time T7 and the noise signalis held in the accumulation capacitor unit 431 a. After that, when thecontrol signals ΦTXA(j) and ΦTXB(j) become H at time T8, the electriccharges in the PDs 421 a and 421 b are transferred to an FD region 423.Since the electric charges in the two PDs 421 a and 421 b aretransferred to the same FD region 423, the signals to which the electriccharges in the two PDs 421 a and 421 b are added (optical signal of onepixel+noise signal) are output to the vertical signal line 427.

When the control signals ΦTXA(j) and ΦTXB(j) become L at time T9. Afterthat, when the control signal ΦTS becomes H at time T10, the transferswitch 430 b is turned on to transfer the signals on the vertical signalline 427 (optical signal of one pixel+noise signal) to the accumulationcapacitor unit 431 b.

The control signal ΦTS becomes L at time T11. The optical signal of onepixel+noise signal are held in the accumulation capacitor unit 431 b,and then the row selection signal ΦS(j) becomes L at time T12.

After that, the column selection signals ΦPH from the horizontal scancircuit 304 become H to sequentially turn on the transfer switches 432 aand 432 b in sequence from the first pixel column to the last pixelcolumn. Accordingly, the noise signal from the accumulation capacitorunit 431 a and the optical signal of one pixel+noise signal from theaccumulation capacitor unit 431 b are transferred to the output circuit305 via different horizontal output lines. The output circuit 305calculates the difference between the two horizontal output lines(optical signal of one pixel) and outputs the signal in which thedifference is multiplied by a predetermined gain value. The signalobtained by the addition readout described above will be hereinaftercalled “first additional signal.”

<Division Readout Operation> (Second Readout Operation)

Next, the division readout operation will be explained with reference toFIG. 6. FIG. 6 illustrates timings for reading signals from the pixel inthe j-th row of the imaging element 100 by the division readoutoperation. At time T1, the reset signal ΦR(j) becomes H. Subsequently,when the signals ΦTXA(j) and ΦTXB(j) become H at time T2, the PDs 421 aand 421 b of the pixel 20 in the j-th row are reset. Next, when thecontrol signals ΦTXA(j) and ΦTXB(j) become L at time T3, the PDs 421 aand 421 b start charge accumulation. Subsequently, when the rowselection signal ΦS(j) becomes H at time T4, the row selection switch426 is turned on and connected to the vertical signal line 427, and thesource follower amplifier 425 enters operational state.

After the reset signal ΦR(j) becomes L at time T5, when the controlsignal ΦTN becomes H at time T6, the transfer switch 430 a is turned onto transfer the signal after reset cancel on the vertical signal line427 (noise signal) to the accumulation capacitor unit 431 a.

Next, the control signal ΦTN becomes L and the noise signal is held inthe accumulation capacitor unit 431 a at time T7. After that, when thesignal ΦTXA(j) becomes H at time T8, the electric charges in the PD 421a are transferred to the FD region 423. Since the electric charges inone of the two PDs 421 a and 421 b (the electric charges of the PD 421 ain this example) are transferred, only the signal corresponding to theelectric charges in the PD 421 a is output to the vertical signal line427.

When the control signal ΦTXA(j) becomes L at time T9 and then thecontrol signal ΦTS becomes H at time T10, the transfer switch 430 b isturned on to transfer the signals on the vertical signal line 427(optical signal of one PD+noise signal) to the accumulation capacitorunit 431 b. Next, the control signal ΦTS becomes L at time T11.

After that, the column selection signals ΦPH from the horizontal scancircuit 304 become sequentially H to turn on the transfer switches 432 aand 432 b in sequence from the first pixel column to the last pixelcolumn. Accordingly, the noise signal from the accumulation capacitorunit 431 a and the optical signal of one PD+noise signal from theaccumulation capacitor unit 431 b are transferred to the output circuit305 via different horizontal output lines. The output circuit 305calculates the difference between the two horizontal output lines(optical signal of one PD) and outputs the signal in which thedifference is multiplied by a predetermined gain value. The signalobtained by the readout described above will be hereinafter called“divisional signal.”

After that, the signals ΦTXA(j) and ΦTXB(j) become H at time T12, andthe electric charges in the PD 421 b are transferred to the FD region423 in addition to the electric charges in the PD 421 a. Since theelectric charges in the two PDs 421 a and 421 b are transferred to thesame FD region 423, the signals to which the electric charges in the twoPDs 421 a and 421 b are added (optical signal of one pixel+noise signal)are output to the vertical signal line 427.

Subsequently, when the control signals ΦTXA(j) and ΦTXB(j) become L attime T13 and then the control signal ΦTS becomes H at time T14, thetransfer switch 430 b is turned on to transfer the signals on thevertical signal line 427 (optical signal of one pixel+noise signal) tothe accumulation capacitor unit 431 b.

Next, the control signal ΦTS becomes L at time T15 and the opticalsignal for one pixel+noise signal are held in the accumulation capacitorunit 431 b. Then, the row selection signal ΦS(j) becomes L at time T16.

After that, the transfer switches 432 a and 432 b become H in sequencefrom the first pixel column to the last pixel column by the columnselection signal ΦPH of the horizontal scan circuit 304. Accordingly,the noise signal in the accumulation capacitor unit 431 a and theoptical signal of one pixel+noise signal in the accumulation capacitorunit 431 b are transferred to the output circuit 305 via differenthorizontal lines. The output circuit 305 calculates the differencebetween the two horizontal output lines (optical signal of one pixel)and outputs the signal to which the difference is multiplied by apredetermined gain value. The signal obtained by the readout describedabove will be hereinafter called “second additional signal” todifferentiate from the first additional signal.

The divisional signal corresponding to the PD 421 a is subtracted fromthe second additional signal read as described above to obtain adivisional signal corresponding to the other PD 421 b. The pair ofdivisional signals obtained as described above will be called “focusdetection signals” because it is to be used for focus detection in theembodiment. Then, the focus detection signals are sent to the focussignal processing unit 104. The focus signal processing unit 104performs a publicly known correlation calculation on the focus detectionsignals to determine the phase difference between the signals. Such apair of divisional signals can also be used to measure the objectdistance and the like as well as focus detection.

Alternatively, the series of operations including reset, chargeaccumulation, and signal readout may be performed on the PD 421 a, andthen the same operations may be performed on the PD 421 b to readindependently the signals from the two PDs 421 a and 421 b by one chargeaccumulation operation. The signals read from the PDs 421 a and 421 b bytwo times can be added up to obtain the second additional signal.

In the foregoing example, two photoelectric conversion units areprovided. However, embodiments are not limited to this example but aplurality of photoelectric conversion units capable of outputtingsignals with parallax may be provided. For example, FIG. 2B illustratesa layout of four PDs in one pixel. The pupil-division PDs 22 a, 22 b, 22c, and 22 d correspond to the micro lens 22. Images different in phasecan be entered into the PDs 22 a, 22 b, 22 c, and 22 d, and signals canbe read separately from the PDs 22 a, 22 b, 22 c, and 22 d.Alternatively, signals may be additionally read from the PDs 22 a, 22 b,22 c, and 22 d, then signals may be additionally read from the PDs 22 aand 22 b, and finally signals may be read from the PDs 22 c and 22 d.Accordingly, the phase differences in the horizontal direction can becalculated. Still alternatively, signals may be additionally read fromthe PDs 22 a, 22 b, 22 c, and 22 d, then signals may be additionallyread from the PDs 22 a and 22 c, and finally signals may be read fromthe PDs 22 b and 22 d. Accordingly, the phase differences in thevertical direction can be calculated. The signals obtained from the PDscan be used for phase difference detection and the like.

Increasing the number of PDs can obtain signals for phase differencedetection and the like by using a method similar to the foregoing one.Briefly describing, signals from n1 PDs are first additionally read, andsignals from n2 PDs out of the n1 PDs are additionally read. In thiscase, n1>n2 is satisfied. This makes it possible to calculate signalsfrom the remaining PDs and use these signals for phase differencedetection and the like.

As described above, in the division readout operation, when thedivisional signal is read and then the second additional signal is read,the signal output operation is performed twice. This takes longer timebetween the start of the reset operation and the completion of thereadout as compared to the case of performing the addition readoutoperation with only one signal output operation. Accordingly, theoperating frequency range between the signals decreases and flickernoise increases.

In addition, in the case of reading separately signals from the two PDs,the signal for image generation is obtained by reading the single pixelstwice and adding up the signals. That is, the readout noise issuperimposed twice on the signal.

<Control at the Time of Frame Reading>

In the embodiment, a selective drive control is performed as one ofreadout controls to read signals of one frame from the imaging element100. Under the selective drive control, at the time of reading oneframe, division readout is performed on pre-decided lines to acquire thefocus detection signals and the second additional signal, and additionreadout is performed on the other lines to acquire the first additionalsignal. FIG. 7 is a conceptual diagram illustrating the case in whichdivision readout is performed on every pre-decided line as an example.By selectively performing division readout and addition readout by rowwithin a frame, it is possible to perform focus detection in the phasedifference system in arbitrary positions within the frame (selectivephase difference AF). It is also possible to shorten the time taken forsignal readout and suppress power consumption as compared to the case inwhich division readout is performed on the entire screen. It is furtherpossible to obtain an image of one frame from the first additionalsignal and the second additional signal.

FIG. 7 illustrates an example of performing division readout on everypre-decided row, but embodiments are not limited to this. For example, atarget region of the focus control may be set from an image signal bythe use of an image analysis function such as face detection or based onan instruction from the user, and rows to be superimposed on the targetregion of the focus control may be set as whole lines in which thedivision readout operation is to be performed. Alternatively, only aposition to be superimposed on the target region of the focus controlmay be set as a region in which the division readout operation is to beperformed.

As described above, the first additional signal and the secondadditional signal are digitized by the CDS/AGC circuit 102 and then sentto the sensor correction unit 103 where the signals are corrected insensitivity failure, shot noise, dark current, variations incharacteristics of the column amplifier. The CDS/AGC circuit 102 doesnot eliminate the difference in noise level resulting from thedifference between the signal readout operations. Then, the imagesignals corrected by the sensor correction unit 103 are input into thefirst NR processing unit 105.

<The Process by the First NR Processing Unit 105>

The first NR processing unit 105 receives from the control unit 109 asignal to differentiate between the addition readout and the divisionreadout, and performs a filtering process on the image signals outputfrom the sensor correction unit 103 according to the received signal. Byperforming this filtering process, the first NR processing unit 105makes signal level correction to suppress the influence of thedifference in noise components resulting from the difference between theaddition readout and the division readout.

FIG. 8 describes a flowchart of the process by the first NR processingunit 105.

At step S801, the first NR processing unit 105 acquires image signals ofa focused pixel and reference pixels for filtering necessary in thefiltering process from the output of the sensor correction unit 103, andmoves to step S802. The reference pixels are neighboring pixels includedin the range of N×N (N denotes an integer) pixels, where the focusedpixel is the center of the range.

At step S802, the first NR processing unit 105 determines whether thefocused pixel was subjected to the addition readout or the divisionreadout according to the signal output from the control unit 109, andswitches processes depending on the judgment. When the focused pixel wassubjected to the division readout, the first NR processing unit 105moves to step S803, or when the focused pixel was subjected to theaddition readout, the first NR processing unit 105 moves to step S805.

Of the signals included in the image signals output from the sensorcorrection unit 103, the signals of the pixels obtained by the divisionreadout have more noise components than those of the signals of thepixels obtained by the addition readout. Therefore, at step S803, thefirst NR processing unit 105 performs the filtering process only on thepixels subjected to the division readout to lower the noise levelsincluded in the signals of the pixels obtained by the division readoutto be equivalent to those of the signals of the pixels obtained by theaddition readout. Then, the first NR processing unit 105 moves to stepS804.

At step S804, the first NR processing unit 105 outputs the signal of thefocused pixel having undergone the filtering process at step S803, andterminates the process.

At step S805, since the focused pixel is a pixel that was subjected tothe addition readout, the first NR processing unit 105 does not performthe filtering process on the focused pixel. Accordingly, the first NRprocessing unit 105 outputs the signal of the focused pixel receivedfrom the sensor correction unit 103 at the original signal level, andterminates the process.

Next, the filtering process by the first NR processing unit 105 will beexplained with reference to FIG. 9.

FIG. 9A illustrates part of an input image in which the rows indicatedwith signs y1, y2, y4, and y5 are the lines having undergone theaddition readout, and the row indicated with sign y3 is the line havingundergone the division readout. The numbers in the pixel positionsrepresent the signal levels of the pixels.

FIG. 9B illustrates pixels in the center of d22 referred to in thefiltering process. In FIG. 9B, total nine pixels in the region of threehorizontal pixels by three vertical pixels are referred to in thefiltering process. In the filtering process, average value processing iscarried out such that the focused pixel is compared in signal level tothe surrounding reference pixels, and only the signal levels with theabsolute values of differences from the focused pixel equal to or lowerthan a pre-decided threshold TH are used. The comparison may be disabledby setting the threshold TH to an infinite value. The filtering processcan be expressed by the equation as follows:

$\begin{matrix}\left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack & \; \\{{{sum} = {\sum\limits_{{x = 1},{y = 1}}^{{x = 3},{y = 3}}{{SUM}\left( {{{value}\mspace{11mu} ({dxy})},{{value}\mspace{11mu} \left( {d\; 22} \right)}} \right)}}}{{count} = {\sum\limits_{{x = 1},{y = 1}}^{{x = 3},{y = 3}}{{COUNT}\left( {{{value}\mspace{11mu} ({dxy})},{{value}\mspace{11mu} \left( {d\; 22} \right)}} \right)}}}{{output} = {{sum}/{count}}}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

In (Equation 1), value(dxy) represents the signal level of a pixel dxypositioned in an x-th column and a y-th row, and value (d22) representsthe signal level of the pixel d22 as the focused pixel positioned in thesecond column and the second row. In addition, SUM function and COUNTfunction in (Equation 1) are as follows:

$\begin{matrix}\left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack & \; \\{{{SUM}\left( {{ref},{center}} \right)} = \left\{ {{\begin{matrix}{ref} & \left( {{{{ref} - {center}}} < {TH}} \right) \\0 & \left( {{{{ref} - {center}}} \geq {TH}} \right)\end{matrix}{{COUNT}\left( {{ref},{center}} \right)}} = \left\{ \begin{matrix}1 & \left( {{{{ref} - {center}}} < {TH}} \right) \\0 & \left( {{{{ref} - {center}}} \geq {TH}} \right)\end{matrix} \right.} \right.} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$

In (Equation 2), ref represents the signal level of the reference pixel,and center represents the signal level of the focused pixel. Theabsolute value of difference in signal level between the reference pixeland the focused pixel is compared to the threshold to decide the outputsof the SUM function and the COUNT function.

FIG. 9C illustrates the result of the filtering process by the(Equation 1) on the input data illustrated in FIG. 9A with the thresholdTH of the filtering process set to 20. Since the line y3 is the linehaving undergone the division readout, the first NR processing unit 105performs the filtering on the signals of the pixels (x1, y3) to (x5, y3)to reduce the noise levels of the line having undergone the divisionreadout. Since the lines indicated with signs y1, y2, y4, and y5 are therows having undergone the addition readout, the first NR processing unit105 does not perform the filtering process but outputs the input signalsas they are.

The foregoing filtering process is an example. Besides the foregoingprocess, the noise levels can be reduced by the use of a processing unitsuch as an filter.

An example of filter for the reference region of three horizontal pixelsby three vertical pixels will be explained below. The number of pixelsmay be changed as necessary.

When the pixels to be referred to by the filter are set as the onesillustrated in FIG. 9B and filter coefficients of the filtering as theones illustrated in FIG. 9D, the filter can be expressed by thefollowing equation:

$\begin{matrix}\left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack & \; \\{{{output} = {{{value}\left( {d\; 22} \right)} + {\frac{1}{\sum\limits_{{x = 1},{y = 1}}^{{x = 3},{y = 3}}{axy}}*{\sum\limits_{{x = 1},{y = 1}}^{{x = 3},{y = 3}}{{axy}*{{eps}\left( {{{value}({dxy})} - {{value}\left( {d\; 22} \right)}} \right)}}}}}}{{{eps}(x)} = \left\{ \begin{matrix}0 & \left( {{2ɛ} < x} \right) \\{{- x} + {2ɛ}} & \left( {ɛ < x \leq {2ɛ}} \right) \\x & \left( {{- ɛ} < x \leq ɛ} \right) \\{{- x} - {2ɛ}} & \left( {{{- 2}ɛ} < x \leq {- ɛ}} \right) \\0 & \left( {{{- 2}ɛ} \leq x} \right)\end{matrix} \right.}} & \left( {{Equation}\mspace{14mu} 3} \right)\end{matrix}$

In (Equation 3), value(dxy) represents the signal level of a pixel dxypositioned in an x-th column and a y-th row as in the (Equation 1). FIG.9E illustrates a graph of function eps(x). In the ε filter, thedifference value between the focused pixel and the reference pixel isvaried by processing with the ε function eps(x). When the difference isequal to or more than a specific value, the contribution ratio of thepixel to the filtering is decreased. In Equation 3, when the differenceis equal to or more than a specific value, the value by which the filtercoefficient axy is multiplied is decreased. The ε function is an exampleand the tilt of the straight line to dampen the difference value may bechanged.

FIG. 9F describes the filter coefficients of filtering, and FIG. 9Gdescribes the results of filtering by the ε filter in which the value ofε is 15.

Also in the case of using the ε filter, the line y3 is the line havingundergone the division readout. Accordingly, the first NR processingunit 105 performs the filtering process on the signals of the pixels(x1, y3) to (x5, y3) to reduce the noise levels of the lines havingundergone the division readout. Since the lines indicated with signs y1,y2, y4, and y5 are the lines having undergone the addition readout, thefirst NR processing unit 105 does not perform the filtering process butoutputs the input signals as they are.

As described above, the first NR processing unit 105 performs thefiltering process only on the lines having undergone the divisionreadout with more noises than those of the lines having undergone theaddition readout. At that time, it is necessary to adjust the noiselevels after the reduction to be close to the noise levels of the rowshaving undergone the addition readout. Accordingly, the first NRprocessing unit 105 performs the filtering process after adjusting thenumber of pixels to be referred to in filtering and the threshold offiltering. In this case, either or both of the following adjustments areperformed: the one that the number of pixels to be referred to infiltering is more increased as the noise levels of the rows havingundergone the addition readout are higher; and the one that thethreshold of filtering is more increased as the noise levels of the rowshaving undergone the addition readout are higher. Thus, the noise levelsafter the reduction are adjusted.

Further, in the case of using the ε filter, the degree of noisereduction can be adjusted by adjusting the filter coefficients to changethe filter characteristics.

In addition, since the first NR processing unit 105 performs thefiltering process only on the rows having undergone the divisionreadout, the signal processing unit 106 positioned at the stagesubsequent to the first NR processing unit 105 does not need to takeinto account the differences in the noise level due to the difference inthe reading methods. That is, the signal processing unit 106 can usecontinuously the method of noise reduction process that was used beforethe introduction of the technique of entirely applying pupil divisionand selectively using the addition readout and the division readout.Accordingly, the provision of the first NR processing unit 105eliminates advantageously the need for making changes to thespecifications of the circuit arranged at the stage subsequent to thefirst NR processing unit 105.

The foregoing description has been given as to an example of filteringand the present embodiment should not be limited to the foregoingdescription of filtering. The first NR processing unit is intended tobring the noise levels of the lines having undergone the additionreadout and the noise levels of the rows having undergone the divisionreadout closer to each other, and the first NR processing unit can bemodified in various manners as far as the foregoing intention isaccomplished. For example, the first NR processing unit may performfiltering not only on the lines having undergone the division readoutbut also on the lines having undergone the addition readout. However,the degree of noise reduction of filtering on the lines having undergonethe division readout is made higher than that on the rows havingundergone the addition readout. Accordingly, after the filtering, thenoise levels of the rows having undergone the addition readout and thenoise levels of the rows having undergone the division readout becomecloser to each other.

<Process by the Second NR Processing Unit 1061>

The second NR processing unit 1061 performs a noise reduction process byfiltering or the like on all the image signals output from the first NRprocessing unit 105, regardless of whether the pixels having undergonethe addition readout or the pixels having undergone the divisionreadout.

The method of the filtering process by the second NR processing unit1061 may be the same as that of the filtering process by the first NRprocessing unit 105. Therefore, detailed description thereof will beomitted.

However, as described above, the first NR processing unit 105 performsthe filtering process on the lines in which the division readout isperformed, whereas the second NR processing unit 1061 performs theprocess on the lines in which the addition readout is performed and therows in which the noise level is reduced to be equivalent to the noiselevel of the rows in which the addition readout is performed.Accordingly, the second NR processing unit 1061 performs the process onthe pixels lower in noise level than the pixels on which the first NRprocessing unit 105 performs the process. Therefore, the threshold setby the second NR processing unit 1061 is preferably smaller than thethreshold set by the first NR processing unit 105.

Second Embodiment

In a second embodiment, the process by the second NR processing unit1061 is different from that in the first embodiment. The otherconditions, the method of PD readout, the configuration of the imageprocessing apparatus, the process at the time of frame readout, and theprocess by the first NR processing unit 105 are the same as those in thefirst embodiment and thus descriptions thereof will be omitted.

FIG. 10 is a schematic block diagram of the second NR processing unit1061 according to the second embodiment.

The second NR processing unit 1061 first saves an image signal outputfrom the first NR processing unit 105 in a memory 1003.

Further, a low pass filter (LPF) processing unit 1011 performs an LPFprocess on the image signal output from the first NR processing unit105, and a reduction processing unit 1012 performs a reduction processof the size of the image signal and saves the reduced image signal in amemory 1013.

Moreover, an LPF processing unit 1021 performs an LPF process on theimage signal output from the reduction processing unit 1012, and areduction processing unit 1022 performs a reduction process on the imagesignal and saves the reduced image signal in a memory 1023.

The LPF processing unit 1011 and the LPF processing unit 1021 perform aband limitation filtering process to suppress aliasing of the image dueto the reduction process. For example, the band limitation filterperforms the LPF process in the vertical direction, and then performsthe LPF process in the horizontal direction to reduce the amplificationof the high frequency.

The reduction processing unit 1012 and the reduction processing unit1022 perform the reduction process on the image signal that isband-limited and output from the LPF processing unit 1011 and the LPFprocessing unit 1021. In the reduction process, the reduction processingunit 1012 and the reduction processing unit 1022 generate an imagesignal horizontally and vertically half the size of the input imagesignal by dropping one each pixel horizontally and vertically from theinput image signal, and output the same, for example. The output signalis temporarily saved in the memory 1013 or 1023.

Filtering processing units 1004, 1014, and 1024 read the image signalsfrom the memories 1003, 1013, and 1023, and perform the filteringprocess on the read image signals to reduce noise according to theirrespective frequency bands.

In the filtering process, average value processing is carried out suchthat the focused pixel is compared in signal value to the surroundingpixels, and only the pixels with the differences from the signal valueof the focused pixel equal to or lower than a threshold are used. Thisfiltering process is an example and another filtering process may beperformed to suppress noise components by the use of a ε filter, abilateral filter, or the like.

Next, an enlargement processing unit 1026 outputs an image formed byperforming an enlargement process on the image signal output from thefiltering processing unit 1024. In the enlargement process, bilinearinterpolation or the like is performed on the image signal to enlargethe image to the size before the reduction process by the reductionprocessing unit 1022.

Next, a composite processing unit 1015 performs a linear compositeprocess on the image signal output from the filtering processing unit1014 and the image signal output from the enlargement processing unit1026. The composite processing unit 1015 detects the degree of flatnessof the image from the image signal output from the filtering processingunit 1014 and controls adaptively the composite ratio used in the linearcomposite process. When the degree of flatness of the image is high, thecomposite processing unit 1015 adjusts the composite ratio such that theratio of the image signal output from the enlargement processing unit1026 becomes higher, and when the degree of flatness of the image islow, the composite processing unit 1015 adjusts the composite ratio suchthat the proportion of the image signal output from the filteringprocessing unit 1014 becomes higher.

The image signal output from the enlargement processing unit 1026 ismore effective in noise reduction than the image signal output from thefiltering processing unit 1014. In contrast, as for the edge parts, theimage signal output from the enlargement processing unit 1026 is lowerin perceived resolution than the image signal output from the filteringprocessing unit 1014. Accordingly, the composite processing unit 1015detects the degree of flatness and adjusts the composite ratio forlinear composition, thereby making it possible to create an image inwhich the flat parts are effective in noise reduction and the edge partsare high in perceived resolution.

Next, an enlargement processing unit 1016 outputs an image formed byperforming an enlargement process on the image signal output from thecomposite processing unit 1015. In the enlargement process, bilinearinterpolation or the like is performed to enlarge the image to the sizebefore the reduction process by the reduction processing unit 1012.

Next, a composite processing unit 1005 performs linear composition ofthe image signal output from the filtering processing unit 1004 and theimage signal output from the enlargement processing unit 1016. Theprocess by the composite processing unit 1005 is the same as that by thecomposite processing unit 1015 and descriptions thereof will be omitted.

As described above, the second NR processing unit 1061 performs thefiltering process on the size-reduced image and then performs thecomposite process after enlargement, thereby performing NR processing insuch a manner as to enhance the effect of reducing noise in the flatparts and maintain the perceived resolution of the edges.

In the embodiment, the reduction process is performed twice. However,the number of times when the reduction process is performed is notlimited to this but may be changed. In addition, the filteringprocessing units 1004, 1014, and 1024 are not essential but may beomitted. This is because the effect of noise reduction can be obtainedto some degree simply by compositing the image signals lowered infrequency than the original image signals.

As explained above, in the embodiment, the first NR processing unit 105performs the filtering process before the second NR processing unit 1061performs the LPF process in the vertical direction. Without thefiltering process by the first NR processing unit 105, performing theLPF process would spread the influence of noise components included inthe signals of the rows having undergone the division readout to theother lines. In the embodiment, the first NR processing unit 105performs the filtering process only on the lines having undergone thedivision readout at the stage preceding the second NR processing unit1061, thereby allowing the second NR processing unit 1061 to perform theLPF process regardless of the pixels having undergone the additionreadout or the pixels having undergone the division readout.

In the first and second embodiments, the first NR processing unit 105performs the process after the process by the sensor correction unit103. However, the present embodiment is not limited to this but theprocessing order may be changed according to the contents of theprocesses.

Third Embodiment

In a third embodiment, the control unit 109 can switch between the modeof switching the readout operation by line and the mode of performingthe division readout on all the pixels in the effective region 301 a ofthe pixel section 301 explained above in relation to the first andsecond embodiments. The processing speed can be higher in the case ofperforming the division readout only in part of the region than in thecase of performing the division readout in the entire region. On thecontrary, in the case of performing the division readout in the entireregion, it is possible to obtain the focus detection signals from theentire region and perform effectively image processing (for example,blurring performance) taking range information into account.Accordingly, it can be conceived that the division readout is performedin part of the region in a moving image capturing mode or a high-speedcontinuous shooting mode in which a higher processing speed is required,and the division readout is performed in the entire region in othershooting modes (for example, a still image capturing mode).

When the still image capturing mode is selected, the control unit 109performs a control to subject all the pixels in the effective region 301a of the pixel section 301 to the division readout, and stops thefiltering process by the first NR processing unit 105. The image signalsfrom the sensor correction unit 103 may be entered into the signalprocessing unit 106 bypassing the first NR processing unit 105, or thefirst NR processing unit 105 may have a path through which the imagesignals input into the first NR processing unit 105 can be outputdirectly.

However, the noise levels of the signals of the pixels obtained by thedivision readout are higher than the noise levels of the signals of thepixels obtained by the addition readout, and therefore the noise levelsof the image signals input into the signal processing unit 106 arehigher than those in the first embodiment. Accordingly, it is necessaryto control the correction levels by adjusting the filtering asappropriate and adjusting the cutoff frequencies of the LPF processingunit 1011 and the LPF processing unit 1021 as appropriate. By making theforegoing adjustments, it is possible to perform the noise reductionprocess suited to the signals obtained by the division readout.

Another Embodiment

The present embodiment can also be achieved such that a programimplementing one or more of the functions in the foregoing embodimentsis supplied to a system or an apparatus via a network or a recordingmedium, and the program is read and operated by one or more processorsin a computer of the system or the apparatus. In addition, the presentembodiment can also be achieved by a circuit implementing one or more ofthe functions (for example, ASIC).

Other Embodiments

Embodiment(s) can also be realized by a computer of a system orapparatus that reads out and executes computer executable instructions(e.g., one or more programs) recorded on a storage medium (which mayalso be referred to more fully as a ‘non-transitory computer-readablestorage medium’) to perform the functions of one or more of theabove-described embodiment(s) and/or that includes one or more circuits(e.g., application specific integrated circuit (ASIC)) for performingthe functions of one or more of the above-described embodiment(s), andby a method performed by the computer of the system or apparatus by, forexample, reading out and executing the computer executable instructionsfrom the storage medium to perform the functions of one or more of theabove-described embodiment(s) and/or controlling the one or morecircuits to perform the functions of one or more of the above-describedembodiment(s). The computer may comprise one or more processors (e.g.,central processing unit (CPU), micro processing unit (MPU)) and mayinclude a network of separate computers or separate processors to readout and execute the computer executable instructions. The computerexecutable instructions may be provided to the computer, for example,from a network or the storage medium. The storage medium may include,for example, one or more of a hard disk, a random-access memory (RAM), aread only memory (ROM), a storage of distributed computing systems, anoptical disk (such as a compact disc (CD), digital versatile disc (DVD),or Blu-ray Disc (BD)™), a flash memory device, a memory card, and thelike.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2016-005346, filed Jan. 14, 2016 which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. An imaging apparatus comprising: an imagingelement including a plurality of pixels configured to have a pluralityof photoelectric conversion units for one micro lens; a readout unitconfigured to perform a first readout operation on pixels included in afirst region of the imaging element to read signals according toaccumulated electric charges, and perform a second readout operation,different from the first readout operation, on pixels included in asecond region different from the first region to read signals accordingto accumulated electric charges; and a signal processing unit configuredto make a correction for reducing noise levels to the signals readout bythe readout unit, wherein the signal processing unit makes thecorrection to the signals acquired from the first region such thatdifferences between noise levels included in the signals read by thefirst readout operation and the second readout operation become smallerafter the correction by the signal processing unit.
 2. The imagingapparatus according to claim 1, wherein, as part of making a correctionfor reducing noise levels, the signal processing unit does not make thecorrection to the signals acquired from the second region.
 3. Theimaging apparatus according to claim 2, wherein the signal processingunit is a first signal processing unit, the imaging apparatus furthercomprising a second signal processing unit configured to make acorrection for reducing noise levels to the signals acquired from thefirst region, after the correction by the first signal processing unit,and to the signals acquired from the second region.
 4. The imagingapparatus according to claim 3, wherein at least one of the first signalprocessing unit and the second signal processing unit makes a correctionto signal level of a focused pixel as a processing target of thecorrection based on signal levels of pixels that are neighboring pixelsto the focused pixel.
 5. The imaging apparatus according to claim 4,wherein a number of the neighboring pixels are to be compared to thesignal level of the focused pixel, and wherein, as the noise levelincluded in the signal of the focused pixel is higher, at least one ofthe first signal processing unit and the second signal processing unitincreases the number of the neighboring pixels to be compared to thesignal level of the focused pixel.
 6. The imaging apparatus according toclaim 4, wherein at least one of the first signal processing unit andthe second signal processing unit calculates differences between thesignal level of the focused pixel and the signal levels of theneighboring pixels and, by use of the signal levels of the neighboringpixels with differences smaller than a predetermined threshold, makesthe correction to the signal level of the focused pixel.
 7. The imagingapparatus according to claim 6, wherein, as the noise level included inthe signal of the focused pixel is higher, at least one of the firstsignal processing unit and the second signal processing unit increasesthe predetermined threshold.
 8. The imaging apparatus according to claim3, wherein the noise levels included in the signals read by the firstreadout operation are higher than the noise levels included in thesignals read by the second readout operation.
 9. The imaging apparatusaccording to claim 8, wherein the second signal processing unit performsa process by a low pass filter on the signal of the focused pixel. 10.The imaging apparatus according to claim 8, wherein the readout unit iscapable of setting a first mode in which the first readout operation isperformed on the pixels included in the first region and the secondreadout operation is performed on the pixels included in the secondregion, and a second mode in which the second readout operation isperformed on the pixels included in the first region and the secondregion, and wherein the second signal processing unit changes a level ofthe correction between a second mode case in which the second readoutoperation is performed on the pixels included in the first region andthe second region and a first mode case in which the first readoutoperation is performed on the pixels included in the first region andthe second readout operation is performed on the pixels included in thesecond region.
 11. The imaging apparatus according to claim 1, whereinthe first readout operation is an operation of reading signals pluraltimes according to electric charges accumulated in the plurality ofphotoelectric conversion units included in the pixels in the firstregion, and wherein the first readout operation includes a firstoperation to read signals according to electric charges accumulated inpredetermined ones of the plurality of photoelectric conversion unitsand a second operation to read signals according to electric chargesaccumulated in photoelectric conversion units different from thepredetermined photoelectric conversion units without reading signalsaccording to electric charges accumulated in the predeterminedphotoelectric conversion units.
 12. The imaging apparatus according toclaim 11, wherein the second readout operation is an operation to readadditionally signals according to electric charges accumulated in theplurality of photoelectric conversion units included in the pixels inthe second region.
 13. The imaging apparatus according to claim 12,wherein the readout unit sets by row a position of the first regionwhere the first readout operation is performed and a position of thesecond region where the second readout operation is performed.
 14. Theimaging apparatus according to claim 13, wherein the readout unitchanges the position of the first region where the first readoutoperation is performed and the position of the second region where thesecond readout operation is performed.
 15. The imaging apparatusaccording to claim 14, further comprising a setting unit configured toset a position of a focus detection region for detecting a focus of anobject, wherein the positions of the first region and the second regionare changed according to the position of the focus detection region. 16.The imaging apparatus according to claim 1, wherein the readout unit iscapable of setting a first mode in which the first readout operation isperformed on the pixels included in the first region and the secondreadout operation is performed on the pixels included in the secondregion, and a second mode in which the second readout operation isperformed on the pixels included in the first region and the secondregion.
 17. The imaging apparatus according to claim 16, wherein, whenthe readout unit performs the second readout operation on the pixelsincluded in the first region and the second region, the signalprocessing unit does not make the correction.
 18. An imaging method foran imaging apparatus having an imaging element including a plurality ofpixels configured to have a plurality of photoelectric conversion unitsfor one micro lens, the imaging method comprising: performing a firstreadout operation on pixels included in a first region of the imagingelement to read signals according to accumulated electric charges, andperforming a second readout operation, different from the first readoutoperation, on pixels included in a second region different from thefirst region to read signals according to accumulated electric charges;and making a correction for reducing noise levels to the readoutsignals, wherein making the correction includes making the correction tothe signals acquired from the first region such that differences betweennoise levels included in the signals read by the first readout operationand the second readout operation become smaller after the correction.19. A non-transitory computer-readable storage medium storing a programto cause an imaging apparatus to perform an imaging method, wherein theimaging apparatus includes an imaging element including a plurality ofpixels configured to have a plurality of photoelectric conversion unitsfor one micro lens, the imaging method comprising: performing a firstreadout operation on pixels included in a first region of the imagingelement to read signals according to accumulated electric charges, andperforming a second readout operation, different from the first readoutoperation, on pixels included in a second region different from thefirst region to read signals according to accumulated electric charges;and making a correction for reducing noise levels to the readoutsignals, wherein making the correction includes making the correction tothe signals acquired from the first region such that differences betweennoise levels included in the signals read by the first readout operationand the second readout operation become smaller after the correction.20. An imaging apparatus, comprising: an imaging element including aplurality of pixels configured to have a plurality of photoelectricconversion units for one micro lens; a readout unit configured toperform a first readout operation on pixels included in a first regionof the imaging element to read signals according to accumulated electriccharges, and perform a second readout operation, different from thefirst readout operation, on pixels included in a second region differentfrom the first region to read signals according to accumulated electriccharges; and a first signal processing unit and a second signalprocessing unit configured to make a correction for reducing noiselevels to the signals read by the readout unit; wherein the first signalprocessing unit makes the correction to the signals acquired from thefirst region, and wherein the second signal processing unit makes thecorrection to the signals acquired from the first region, after thecorrection by the first signal processing unit, and to the signalsacquired from the second region.